This invention relates to the continuous extrusion of a puncture sealant used in the first step of the construction of a tire carcass which, upon being cured in a conventional tire curing press, results in a self-sealing pneumatic tire. Uncured sealant is a cohesive sticky elastomeric mass which is readily deformed but will not flow even at elevated temperatures unless under pressure. After being cured, the sealant remains a cohesive and sticky elastomeric mass which is not so readily deformed and will not flow at elevated temperature and pressure unless both are relatively high. Those skilled in the art understand that puncture sealant stock, cured or uncured, refers to a highly viscous, plastic mass of rubbery adhesive which is essentially non-flowable at a temperature below about 250.degree. F. and a pressure of about 5 psig.
More specifically, this invention relates to a process for extruding a generally flat strip of sealant from an extruder to which is supplied a stock of a sealant recipe which consists essentially of a vulcanizable ("curable") mixture of a high molecular weight (mol wt) elastomer, a low mol wt elastomer, a curing agent ("curative") and a processing aid selected from a tackifier and a homogenizer. The strip is required to be essentially uniform in thickness, less than about 10 mm thick, and at least 10 times as wide as it is thick. The curative is effective at a temperature above 250.degree. F.
The sealant stock is taken from storage and preheated before it is supplied to the extruder as a hot plastic mass which does not flow and cannot be poured from one container to another even at 212.degree. F. Moreover, each component of the sealant recipe strongly adheres, at ambient or elevated temperature, to any surface, and in particular each component has great adhesive affinity for the other. Yet it is essential that this stock be extruded to obtain the desired uniformly flat strip.
Further, because the strip (or, extrudate) must be of essentially uniform thickness, for best results, we have found that the sealant stock must be extruded with a screw-type extruder; and, because the rate at which the strip is discharged from the extruder is relatively sensitive to the conditions of extrusion, such as temperature of the stock, the screw speed, the internal pressure in the barrel of the extruder, etc., such minor but acceptable variations in thickness as may occur, must be controlled by the speed of the endless belt conveyor onto which the strip is continuously discharged. We have found that control of thickness by conveyor speed permits relatively speedy results. The alternative, namely changing extrusion conditions requires a relatively long time before any change in extrusion conditions produces results in the extruded flat strip.
Recognizing that a screw-type extruder is prone to cavitation if used to extrude a cohesive, non-flowable, plastic stock, it was particularly surprising that an intermittent pulsing pressure, maintained on the stock at the intake of the extruder, would overcome cavitation of the extruder and furnish a flat uniform strip of sealant.
Still further, we found it essential to coat the strip with a detackifying compound ("detackifier") or the extrudate cannot be parted from a surface onto which it is deposited; nor can a non-detackified strip be parted from another such strip on which it is placed, as it would have to be parted from preselected lengths of strips stacked for later use; or, the strip parted from itself, as when a continuous strip is rolled up as it is removed from the endless belt. Since we have found that only a polymeric alkenyl alcohol is able to provide the desired detackifying function, and it can, in practice, only be uniformly applied to the surfaces of the strip as an aqueous solution, it is essential that the water be removed before the strip is stored, or readied for use in the tire building operation.